Radio reflection direction and distance determining system



Dec. 30, 1947. A D. BLUML'EIN RADIO REFLECTIONDIRECTION AND DISTANCE DETERMINING- SYSTEM Filed Ju'ne 15, 1942 Patented Dec. 30, 1947 RADIO REFLECTION DIRECTION AND vDISTANCE DETERMINING'SYSIIEM Alan Dower'Blumlein, Ealing, Englandfsassig-nor `to Electriczl Musical Industries Limited,Hayes, Middlesex, England, avg-.companys oftiGr-eat Britain Application June 13, 1942,-SerialNo.-446;971 In GreatBritain Decembelwl,` 1939 @Section 1, PublicLaw 690,*Augustv8f1946 .Patent expires December4,.1959

10 Claims.

switching means and 'means for feeding said dif- -20 Jenn Q ffeaid-seeond train 0f., recurrent signals,

ferentportiongfromlsaidswitching means to said retaining-deviceso as todevelop therein a derived-signal havingJ substantially the vsame Waveformas atleastapartof the-waveform of said recurrent signals. Y

' Preferablv the-'timing ofthe Vtime intervals during whichl said d-iierentgportions are selected changes continuously 'in-the same4 sense during a time. period long compared With'the. time of recurrence -of'gsaid recurrent signals. If random interference is superimposed on said train of signals, theysame ora similanportionof said re- Vurrent signalsrmay'beselected from a plurality ofsignals and said plurality of selectedportions `may Ibe integrated sofasto increase the minimum ratio of the amplitude .of said recurrent signals and said randominterference.

.Said recurrent signals may be oscillatory signalsjhaving `substantially `the, same lenvelope waveform andsaidselected portions may then be fedto. a retainingdevice Whiohis such .that osvcillationsare 'setuptherein by said portions in sllCh. mannerI that the oscillations .s et up in said device, bysaid selected portions Vadd substantially in phase. If desired, said oscillatorysigna-ls may be changed 130,` a ...d ierentVr frequency priorto said selectionV and .integration, and said integrated signals may be rectiedand iurtherintegrated after rectieation. v

According .to another feature of.. thevinvention there islfprovi'ded-apparatus for measuring the .diflerence of timing-between `twoftrains `ofre- `.currentsignals,.,the.= Signals of each train :having substantially athen-sauleVv Waveform, Acomprising a `iirst -z fsisnaleretainina .device ,a inst .switching .55

2 means fpr'. seleeiing at time intervals. diereni .portions of ,different recurrent signals, means `for feedingjjthejrst ofmsaid-trains of signals to said 'iirst svvitchi-ngmeans,` means fonfeeding said different HA,portions jgfronrissaid `iirst switching means to said,rst retainingdevice soV as `to develop therein la, .derived ,'signalihavng. substantiallylthe samemwaveform as atgfleasta part Aoi the .waveform oisaidgiirst recurrent signals, Va second signal retaining device, ga Vsecond v,switching means for selecting at., time .intervals different portions .ofsdifferent reciirrent.,vsignals ofr thesecond train of.v signals, means 4, fore-. feeding y. the said different portions of saidsecondtrain of, signals tpfsaid seCQnd-swihinef means, rnea'lfls` foe-feeding sisnals from ,said second switching means to Asaid second-retainin deviee so as Cto y ievelop therein asesor-.1d delvedsignal :having: substantially.: the .Same Waveform- .as emessi a; part of lthe. waveand means. for-adjusting,andsmeasuring thediff-erenee .ngfltirning-between said, time intervals ddring-,whieh said diilerent lportions of saidarst and secondisignals,trains arev fed to said first-and second-retaining device respectively, whereby Whentheti-mines voisaidtirne intervals are so. adjusted -Lthat saidrstf .and `second derived signals ,havesubstantiallythe'same waveform, the diierence' natiming betweenY said ,intervals V.in substantially equal tothe: diierencexin Itiming zbetween :saidf rstfand Asecond train 4 of signals.

` .Thezfeaturesf'of .a the :invention -abovereferred .toimany 'be Lusedffor A:.letecting or for determiningxthefposition and/or fdistancerof a'source of recurrent signals `having substantially the `same iwave'form.

fThe application of the invention to the detection of, or the determination ofthe positionror 'distanee of -asource `or reflector .of radiation will now be f described by way of; example withl reference tothe` accompanying drawing, which shows ageneral-'schematic 'circuit 'arrangement ofvvthe transmitting. and ,receiving apparatus- .'.Tlie snosition all/iwv` distance oisources 0f ,radiationv lmay be determined byv a method which eenprisesI transmitting regularlyrecurrent *"bursts of ,radidfrequency carrierY waves,. re- ,ceivingithegreectionof such Waves-due to reflectmg r.objects such as. an `.aircraft.upon.. a `number of spaced j.aerials 4and determining the azimuth :and elevation .of. .euch fi-reflecting objeQtbv-measuringfthee dlelele .lnstiming .ofthe .signals de- .rivedtffroinvfr-said aerials. This diierence Iin .timinse-may seither .,befmeasure'd directly on theesie- 3 nals derived from the aerials may be delayed by known amounts so as to equalise their timing. The present invention will now be described as applied to an apparatus of the latter type.

Referring to the drawing, Tr represents a radio frequency transmitter adapted to radiate bursts of carrier wave under the control of the master pulse generator PO. The carrier wave frequency may be 100 megacycles/sec. and the bursts may be 0.5 microsecond in duration and may recur every 200 microseconds, the master pulse generator PO then having a frequency of 5,000 cycles/sec. Preferably, the master pulse generator PO is arranged to drive a subsidiary pulse generator PGI, which has means for controlling the duration of the pulses, and the pulses are then fed to the transmitter Tr over a delay network Dt, which permits the adjustmenty of the timing of the pulses.

If the signal radiated by the transmitter in incident upon a reflecting object, such as a metal aircraft, it will be reflected. Such reflected signals may be received upon three aerials Am, Ay, Az, situated in a horizontal plane at the apices of an equilateral triangle. The received signals are fed from these aerials to three receivers Rx, Ry and Rz respectively of a type which will hereinafter be described in which the timing of said signals may be modied and from the output of these receivers the signals are passed to a shuiiiing device Sh, hereinafter more fully referred to associated with cathode-ray oscillographs OI, O2 and Od, which indicate the relative timing of received signals.

The receivers Rx, Ry and R2 may comprise one more initial stages Xa., Ya, Za, respectively, having a relatively broad pass band of 100 megacyclesil megacycles/sec. which are normally blocked so as not to transmit signals, but which are switched into an operative condition so as to transmit signals by a switching pulse 0.5 microsecond in duration and recurrent at 5,000 cycles/sec. derived from the pulse generator PG2, controlled by the master pulse generator PGI. Such stages may be of any suitable kind and may be supplied with anode voltages greatly in excess of the voltages normally used with the valves employed due to the fact that only intermittent operation is required. In this way the valves have an increased mutual conductance.

The signals passed by said switched stages are then successively heterodyned to lower frequencies and are finally reduced to a frequency which may be 20 kilocycles/sec. and fed to a circuit tuned to 20 kilocycles/sec. and having a pass band of i100 cycles/sec. in which the signals are effectively integrated over 25 successive cycles, whereby, as explained in the specication of copending application No. 446,970 filed June 13, 1942, Pat. No. 2,406,316, dated August 2'7, 1946, granted to A. D. Blumlein and E. L. C. White. The effect of random interference upon said signals is reduced.

Thus, the receiver Rm includes one or more initial switching amplier stages Xa which selectively feed signals from the antenna A to a first mixer-rectier XTI wherein the received signals are heterodyned by oscillations from a first local oscillator LOI. The heterodyned signals are fed through a first integrating or signal retaining band-pass filter XFI to a second mixer-rectifier X72 wherein the signals again are heterodyned by oscillations from a second local oscillator L02. Signals from the second mixer-rectifier XTZ are applied to an integrating tuned oscillatory circuit i or signal retaining device XFZ. The nally integrated signals thence are appliedthrough a gain control device Gx and a third rectifier X3 to the shufiiing device Sh.

The receiver Ry includes similar initial switching amplifier stages Ya responsive to signals from the antenna, Ay, a rst mixer-rectifier YI connected to the rst local oscillator LOI, a first band-pass lter YFI, a second mixer-rectifier Yr2 connected to the second local oscillator LOZ, a second integrating or signal retaining device YFE, a gain control Gy and a third rectifier Yr3 having its output connected to the shuiiiing device Sh.

The receiver Rz is similar to the receivers Rx and Ry and includes the elements Za, ZTI, ZFI, Zr2, ZFZ, GZ, and Z13. Signals applied to the mixer-rectiiiers ZTI and Zr2 are heterodyned respectively by oscillations from the local oscillators LOI and L02. The output of the third rectier Zr3 is applied to the shuiiling device Sh.

The timing of this switching pulse which renders the early stages of such receiver operative is varied in a cyclic manner by means of the pulse modulator PM which varies the timing of the pulses under the control of the saw-tooth waveform generator SG, which may conveniently have a frequency of 25 cycles/sec. The saw-tooth voltage provided by SG, is preferably added in the pulse modulator PM, to saw-tooth pulses derived from PG2. and a pulse is derived whenever the resultant voltage increases beyond a predetermined value. It may thus be arranged that the 5,000 cycles/sec. pulses occur successively later throughout successive periods of 1/25 second, i. e., the timing of the pulses changes continuo-usly in the same sense. The effect of this variation in the timing of the switching pulse is that different portions of successive recurrent signals are selected by the switched stages of the receiver and a signal having substantially the same waveform as the envelope waveform of the recurrent signal is thus reconstituted in the output of the receiver during the time period of the saw-tooth waveform which modifies the timing of the switching pulses, i. e., during a time period long compared with the time of recurrence of the recurrent signals.

It will be appreciated that although the successive selected portions of the signals are not strictly identical due to the continuous change in the timing of the switching pulse, provided that, as in the case in the present example, the change in the selected signal during the effective period of integration is small, the successively selected signal will be similar and will be integrated in the tuned circuit above referred to. The output signal, therefore, has substantially the same waveform as the envelope waveform of the incoming recurrent signal and may be observed on the screen of a cathode-ray oscillograph Od by applying the derived signal to one pair of deflecting plates and a saw-tooth waveform from the generator SG to the other pair of deiiecting plates.

If only a limited portion of the waveform of the recurrent signals is of interest, the variation of the timing of the switching pulse may be localised to the neighbourhood of the timing of that portion of the waveform which it is desired to observe. In the present example, the variation of the timing of the switching pulse may be limited to a range of i microsecond about the timing of the incoming reflected signal.

If the receivers Rx, Ry and Rz are simultaneous'ly-A 'suppliediwithrthe `same1 switching-pulses iinszf.

thelfmanner @referred "to, derived signalsf having substantially the same waveform as the enveloped.-

waveformf of thel` incoming.4 signalswill appear` 'in theirloutputcircuits;` but these derived signals."l will-liavefthe-sanieirelativefphase as th'e'incomin introduced into :the switching pulses' fed to the i different-receivers, the; relative timingof'the de-l the-"ltimingzof.they-signal `received onaerial` Am i. is ahead ;of-.the"timingof the signal received :on r aerialmAyg-:thederived signal .appearing inthe outputi ofV receivers Ra,`- and'Ry'fmay rbe made lto 'i havef-th samewtiming by.' advancing 'the .timing` of th switching '1i-pulses for the receiver ,R,iandl.-

thiscadvance in timing -will -be a direct-measure"4 A of the `diiiferenceein timing between the signals-:-

incident-*upon aerialsAli-andyil The switchingpulses from the pulse modulaton: PMare; therefore; .fed to i the'y receivers-Rit',v l-Ry'.

andvRaover adjustable delay.- networks -Dd, .Dmy,.

Dm, 1Dy` andfDzbywmeans of which relative -tim`f ing-of thepulses-fedto the three receivers may be changed Icy-known amounts. Thel time delay:`

netword Dd is provided to take up the time de lay-of ltheiincoming signals due to their time of transit from vthe transmitter `to the rei'lecting` object and backto the receiver, and-.to provide fora range `01 30 km., the delay available in Dd should be atleast `200. microseconds.

The delay provided by the time delay networks Dx, Dy', Da and `Dry may; of course, be very much less than this-"as f these networks .need only take up the small-delays of a fraction of 'a microseoond due to the'path difference from the reecting object to thei'different aerials.

Thedifference` in timing between the signals received on the aerials Am, Ay, A2 is thus dete'rw mined'by adjusting the delai7 network above -referredto until the timingsof thederived signals in the outputs fof the receivers Rx, Ry and 1ra-are`V made equal. signalsis 'preferably indicated as follows:

The-receiver outputs Ox, Oy and Oz 'are passed- The relative timing of the derived to the shulliing device Sh which produces outV putsfswproportionalLto the sum OrciOyiI-Oz 3 an integrated difference.- DI proportional'fto NOx-Joy) dt; andfanintegrated diierence DZT proportion to wheret represents time; The aerials are preferably50 metres apart, in which case there will be a maximum of 0.166 @microsecond -differenece in timing. -The two dilerence waves- DI 'and D2 will be approximately similar in waveform and phase to the sum wave. Su, but with magnitudes and senses relative to Su, depending on the relativetiming-.of the.Y signals from thefreceivers.

Thelintegrated,diierences DI and DZ'are apav )plied-tc'ithei horizontal deflecting plates of the.,

bearing indicating. oscillographs Ol and O2`and thefsum'" Su" is applied to the vertical" deecting i plates'of these'oscillographs. If the timing of the receiveroutputs is similar vertical lineswilljbe traced;` Any difference in timing will give inclined lines.

The pulses Pfr'and Py are-passed through'delay'f-networks Dai and Dywhich are differentially controlled-to -alter the relative timingof PasandV Pyfso as to make-thee outputs fof receivers'Rrfand "tween the transmitted pulses.

and 'fila Hlizerticalli;ilinesis `therefore obtained. son, oscillograph Ol. The setting oftDztr-.andDyvnovv.l gives the relative-timing."` of the. received` waves-- at the aerials Ananda Ay. Similarly, -the.rela' tivev timing. .of apulseszPat'and v.the :mean .of .Prc and; Y

Py aref-adjusted by;.the1-di1erentially controlled delayiinetworksDxyanduDaso as to give'no tim-" VH ingirdiierenceibetweenrOaandzOrc+zOy so thata y wertical line is obtained atoscillograph O2.' The: 1 settingsf. Vthe;.-delayvcontrols now deiinethe` 1 bearingLof theopulse reflecting object; It is var.V

rangedfthatlfthe"differential .control varies thei';

delay; in iiDzlutwice; Sasjrapidly .asain` Dry, thus; 1:

`always .keeping thefmeanLtil-ning of PaglPy and.. Pa constant; V

If Trvyis the delayof Py! relative: to yPfc andif /LT 2 TZ" ig the-eelayrof Per relative to the-mean offrit-,-V

and Prg/:,:whenboth oscillographs :show vertical.,` traces, :then theyazimuth .angle of .theA reflecting.;y

object is .3

measured in sense' xyz and the elevation angle is cosi1 TxyZ-T? when Tzry and Taare measured vin units of 0.16 microsecond (that is, .theaerial spacing cf,5ometres divided bythe velocity of light).

In .the.above..no.V allowance has b-eenmade. for thevariationof delay between receiver and receiver, whichfmay be allowedforby small preset Y ldelaystinserted in the leads between Dx, Dy, De

and the receivers.v Thse may be adjusted by means of a test signal .radiated .from aerial Ats to be described later.

In order thatasuitable reflection maybefound andthe delay YDd adjusted easily, a Search aerialA'5s and a Search receiver Rs. are provided. This vis similar to the other receivers Rrr, Ry, Re, includes .the circuitcomponentsSa, Srl, SFI, ST2, SF2, Gs and ST3, and shares the same 'heterodyning frequencies as the other receivers.

The pulse Ps which periodicallyswitches on this receiver is, however, derived separately from the pulsing. oscillator PO. The 5,000 C. P. S. wavev A from VPO is passed .through `a phase rotating delvice PhR, which may be similar to a goniometer A tion in, say, 2 seconds;V Thus, every2 secondsV the pulse actuates the receiverRs later and laterv4 duringY the1/5oo0 sec;Y period :so that theoutputvv of Rs traces `out the envelope waveform 4of 'all' the reflected waves received in the intervals be- This output is applied to the vertical deflection plates of a Search oscillograph Os, which is provided with" a long lag iluorescent screen, having a fluorescent delay time of, say 5 seconds. The horizontal deflection plates are connected to a scanning generator Sc which scans thecathode ray across thescreen once for every revolution of PhR. Thus, a picture of the envelope waveforms of alli the vreceived-Y signals is obtained on the screenandz-xiiovementsA of anyv particular. reiiectingobf-fff ject can be detected by the movement ofv the envelope waveform of the signals reflected therefrom on s'aid screen.

In order that the approximate distance of any such refiecting object may be determined on the search oscillograph Os, for example, in order to facilitate setting the distance delay Dd, a marker device Ma is associated with the mechanical drive to phase rotator PhR. This marker is arranged by mechanical contacts to momentarily cut off the beam at, say, ten periods during the revolution, and further to reduce the beam brightness slightly at, say, nine points between each complete cut-off. Thus, the trace on the search oscillograph will be marked out into 100 divisions which for the case considered will each represent 300 metres distance from the transmitting and receiving apparatus.

The test and searching aerial Ats is preferably situated symmetrically between the other three receiving aerials. A test cable TC may be provided which may be of such a length as to provide, say, 4 microseconds delay for a 100 m, c. wave. Such a delay would correspond to a range of 600 metres. Switches Sl, S2 are provided by means of which the transmitter output can be diverted from the transmitting aerial via the test cable to the test aerial. If the apparatus is correctly adjusted such a signal should give readings corresponding to an elevation of 90 and a distance of 600 metres plus the distance from the test aerial to the other receiving aerials less an allowance for the diiference of length of feeders to the test aerial and the transmitting aerial.

The controls to the distance delay Dd and the y and e delays are set to the correct values, and the adjustments made to give vertical lines on the bearing oscillographs and a central pulse on the distance oscillograph. The delays (not shown) in the leads Px, Py, Pz to the receivers Rx, Ry, Rz respectively are adjusted to give vertical bearing lines. The transmitter delay Dd is adjusted to give a centrally placed pulse on the distance oscillograph. The setting of the marking mechanism for the search oscillograph Os is similarly adjusted to bring the observed pulse to the 600 metre mark.

If the receiver gains are not equal, the lines on the bearing oscillograph will appear as ellipses. A control Cl may, therefore, be provided adjacent the oscillograph Ol to adjust the gain of Rx and Ry differentially, Similarly, a knob C2 adjacent O2 adjusts the gain of Rz. The knobs are adjusted to give straight line traces. Further gain adjustment of the receivers is preferably eifected by means of variable attenuators included in the three receivers which are mechanically coupled to vary the gains of the three receivers equally and simultaneously. The search receiver Rs is preferably given a separate gain control and may if desired have an automatically variable control supplied from the scanning generator to make the receiver mo-re sensitive for distant objects. The three bearing receivers may, if desired, have separate automatic volume control to maintain the received pulse amplitude constant.

It will be appreciated that it is desirable to limit the pass band of the receiving apparatus to a minimum in order to reduce the effect of interference. The pass band of the post detector stages may be limited to approximately twice the repetition frequency of the derived signal so as to pass the iundamenta1 and rst harmonic frequencies of this pulse. Thus, if the derived sigg nal is reconstituted every 1/25 sec., the pass band of the post detector stages may be limited to 50 cycles/sec.

The pass band of the pre-detector stages is chiefly determined by consideration of the Doppler eifect due to the motion of the reflecting object, and with the present speed of aeroplanes this pass band cannot safely be limited to less than, say, cycles/sec. or 200 cycles/sec. to allow a tolerance for cumulative inaccuracies in timing frequency stability, etc. If, however, a correction be applied to reduce the frequency shift of the incoming signal due to the motion of the reflecting object, the pass band of the predetector stages may be reduced. Such correction may be applied by known automatic frequency control methods, but is conveniently applied automatically by the phase rotating device PhR associated with the search receiver Rs in the following manner.

As 'has been hereinbefore explained, the distance of the reflecting object is indicated on the distance delay network Dd, and as the object moves this network has to be continually readjusted. The rate of readjustment of this network will be proportional to the velocity of approach of said object and by coupling a device, such as a magneto Mg, with a rotating adjustment element of this network a voltage may be generated which is proportional to the rate of approach of the reflecting object and may be applied to adjust the frequencies of the heterodyning oscillations generated in the local oscillators LOI and L02 of the receivers so as Vto reduce the frequency shift in the final I. F. signal due to the motion of the reflecting object, for example, by changing the capacity or inductance in one of the frequency-determining circuits of said oscillator.

Reflected signals from other reflecting objects which are stationary, or which have a diiferent component of velocity towards or away from the receiver 3, i. e., objects moving relative to the reecting object which it is desired to observe, will have a different frequency from the signals reiiected by said reflecting object and can be rejected by suiciently reducing the passband of the I. F. circuits, or by tuning the I. F. circuits so as to be non-responsive to the intermediate frequency signal derived from the reflected signals received from such objects.

The system for the determination of the position and/or bearing of reflecting objects described above depends upon the measurement of the relative phase of the envelope waveform of the reflected signals, and as this phase measurement becomes ambiguous when the phase difference to be measured exceeds the separation of the aerials necessarily has to be less than that which would give rise to a phase difference ap proaching this value. With the present system, however, this limitation is removed, and difference in timing up to the time separation, in the present example 200 micro-seconds, if successive bursts of the high frequency carrier, may be measured. It is thus possible to provide more than one set of three aerials having different spacings so as to enable the aerials with the greater spacing to be switched into circuit to give more accurate bearings when required.

In order to reduce the effect of oscillator drift, the local oscillations may be derived in known manner by mixing an oscillation derived from the carrier frequency oscillator of the transmitter with an oscillation of the desiredintermediate aisaesif frequency derived ffrom f another` oscillator,u the` frequency lstability,'oflwhich-isl good relative: to. the -pass band l required of -thedesired-intermediate frequency@V For example;V therst heterodyning oscillation maybeobtainedby mix- 51 ing. the carri'er'lfrequenc-y' oscillations --from.the transmitter -vvith`afstable oscillationof -mega=- cycles -per second, the'ffrequency drift `of=.tl'1elat. f ter oscillator being"smal1-withreference-.tof200ekilocycles f perseconda Alternatively;4 the carrierV l0' frequency of the-.transmittermay'be synthesisedf from-a number foflower frequencyoscillationsl including'fthoseV required for the `het'erodyningn stages ofthe receiverw Thus; ifv the' fre-'fl quencies v of the receiver are-to"be*-' 10-' mega 15 cycles/sec.,l 300 *kilocycles-/secf andkilocycles/ secr respectively, and ,the--frequencyof the transe-Y mittedcarrier and the reectedsignar to rbeire-f ceived i's 100 megacy'cles/secr the local* os'cill'atifl'r` frequencies requiredfwill lloe `90 megacyclesf/secr; 20v 9.7megacycles/sec: andA 280='kilocycles/sec.*fre-t spectivelyi Thesefrequencies may Eloe'generatedi by. starting with oscillations-operating at fre-f quencies l f of 20v-and 280l `-kilocyc`lesysecs; vmixing.' these oscillations to' 'give the 4'300 'liilo'cycle'sfsec'r` 25 oscillation; mixing thi's `300 'kilocyclesf-/sec oscillation with a 9.7" 'meg'aoyclesysec oscillation to givea 10 megac'ycle' oscillationv and 'nallyfrnixf lng this l'megacycles oscillationwith a 90 negacycle oscillation to give thelOO megacy'cle "oscilla-Y 30 tion required for the transmitter. Any variation of frequency of these oscillators', exceptingl the 20 kc. oscillations'will'then not change tliefrequency of the final 20'kilocycles/sec. I. F? signal. This arrangement has the furtherV advantage that', since the heterodyning `oscillation-islnot derived from a signalfrequency oscillation, the risk of a slight admiXture of the signal frequency in the heterodyning` frequencyis prevented and interference, which the presence of suchA a corn- 40 ponent would cause, is therefore avoided.

The interfering effect'of a steady carrier frequency in the methods of reception Whichhave been described above `I nay be reduced -by modulating the carrier frequency of the transmitter at a very low frequency. It will be seen that the frequency of the transmittermust not l'change by, more than a small fraction" of 200 cycles/sec; in. 1/5000Y sec., as otherwise successive reect'edlsig' nals will 'not add with' a suflicient degree of accuracy because the received signal may be heter- Y odyned by an oscillation derived from the transmitter 1/5000 sec. after it vhas itself transmitted.- A frequency variation of` 10 cycles/sec. may, how ever,r be` permitted inA 1/5000- sec. andthe frequency of the transmitter'.` may be. modulated over a range'of $2500"cycles/sec.;inaperiod.of-f 1/5I sec. That is to say, the-frequency ofthe -transmitter may -be .increased steadily during-le" sec.l andthen decreased-steadilyduring the-followingn lelsec. and `so on; Thislmay-loe donefbymeansr' of aI mechanically rotated condenser-1 in Yone of the frequency-determining circuits of the transe mitten YFor a steady interfering-signal,.this fre- 65 quency modulation --of\ the transmittedcarrierr Will give freedomffrom yinterferencefrom such a signalfor.`

100 per centv 70 i..e.,- 92 per cent of'feachl-lsecperiod,"- Aslovver.

variati-on of frequency than-that suggestedabove-g may, of course, .becusedfw Y `Although the timingyof the switching pulses 751 in the arrangement `which has been described. above changes continuously in the same sense during-a time period long compared with the time of recurrence of -said recurrent signals so that similar but not necessarily identical portions of successive recurrent signals are selected, it will be appreciated that the porti-ons need not be selected from successive recurrent signals, since the switchingmay be arranged so thatnov portions are selected from some of the recurrent signals in the signal train. Further, the switching may also be arranged so `that the same `portion is first-selected from a plurality of the recurrent signals and then another portion is selected fr-om another plurality of the recurrent signals and so.on,in which case thetiming. ofV the switchingV pulses Will be arranged to changeY discontinuously. It will also be appreciated that it is not necessary to employintegration incarrying out the invention,.as. the derived-signal may be developed by feedingthe selected-signal por.- tions to a device Whichvonly retains a current orvoltage representative of the. amplitudeof'oneA selectedportionuntil the next selected .portion arrives, so that the current or voltagein thedevice is alvvays directly-related to the amplitude of themost recently received selected portion and is unaffected by previous signal. portions. Such a device may comprise acondenser, or if the signal isoscillatory, an oscillatingA circuit.. tuned to the frequency of the signal oscillations, l. so arrangedthat the voltage. or. oscillatory 'current in said condenser .and circuit respectivelydue to each selected portiondisappears when the next selected portion-is fed thereto.V Such a device Willbe termed .a retaining--deviceandthis1 term is to beconstrued-so asA also to include-a device which` retains its venergy'forha period. longer than -the interval between -fsuccessively selected signal 'porti-ons,- -so-thatfintegration takes placedue ito the addition of the -energy due-to each selected portion' to'the energy yremaining.y from 'previously selected^-portions.V Y

It'vvill be' understood that the arrangement ldescribed above lmay -be 'modified toV detecty or de-r termine' the positionwof' submarine sources or re-'- flectors `of supersonicvibrations-in water. In" thiscase short'pulses of supersonic vibrationsvvill naturally be used-instead ofshort'pulses-of radio` frequency radiation, but' the' system will -besimilarin principle.-Y f

Although the invention has been' described above with reference to a system for "thedetec tion or location of reflecting objects, 'it will Vbe" appreciated that it is" broadly Vapplicalolato -the derivation 'of a signal from 'any' trainof `recur-- rent signals having substantially'the same-Wave form and preferably-separated by'equal time-in-` tervals. Said `derived signals VVmay have substane0. tially "the same Waveform' as 'the-Wholefor selected tion-may'lbe'applied both before andfafter rec-l` tiiication.r

WhatI claimfis':

l. Apparatus-for deriving-asigna] from a train.' of 'recurrent signals having substantially thefsameF waveformwhich comprises i-a signalv retaining de-1 vice, switching means for selecting different portions of different recurrent signals. means for feeding said train of signals to said switching means and means for feeding said different portions from said switching means to said retaining device, and means for continuously changing the operational timing of said switching means feeding said retaining device for deriving a signal having substantially the same waveform as at least a part of the waveform of said recurrent signals.

2. Apparatus for deriving a signal from a train of recurrent signals having substantially the same waveform, which comprises a signal integrating device, switching means for selecting at time intervals different portions of different recurrent signals, means for feeding said train of signals to said switching means, means for feeding said different portions from said switching means to said integrating device, and means for continuously changing the timing of said intervals in the same sense during a time period long compared with the time of recurrence of said recurrent signals, whereby there is developed in said integrating device a desired signal having substantially the same waveform as at least a part of the waveform of said recurrent signals and whereby the minimum ratio of the amplitude of said recurrent signals to the amplitude of any random interference superimposed thereon is increased.

3. Apparatus for measuring the difference of timing between two trains of recurrent signals, the signals of each train having substantially the same waveform comprising a first signal retaining device, a first switching means for selecting at time intervals different portions of diierent recurrent signals, means for feeding the first of said trains of signals to said rst switching means, means for feeding said different portions from said first switching means to said rst retaining device so as to develop therein a derived signal having substantially the same waveform as at least a part of the waveform of said first recurrent signals, a second signal retaining device, a second switching means for selecting at time intervals diiferent portions of different recurrent signals of the second train of signals. means for feeding the said different portions of said second train of signals to said second switching means, means for feeding signals from said second switching means to said second retaining device so as to develop therein a second derived signal having substantially the same waveform as at least a part of the waveform of said second train of recurrent signals. and means for adjusting and measuring the difference in timing between said time intervals during which said different portions of said first and second signai. trains are fed to said first and second retaining device respectively, whereby when the timings of said time intervals are so adjusted that said first and second derived signals have substantially the same waveform, the difference in timing between said intervals is substantially equal to the difference in timing between said rst and second train of signals.

4. Apparatus according to claim 3 in which each of said retaining devices comprises an integrating device and means are provided for continuously changing in the same sense said time intervals during a time period long compared with the time of recurrence of said recurrent signals, whereby the minimum ratio of the amplitude of said recurrent signals to the amplitude 12 of any random interference superimposed thereon is increased in each integrating device.

5. Apparatus for deriving a signal from a train of recurrent oscillatory signals having substantially the same envelope waveform which comprises an integrating device adapted to have oscillations set up therein, switching means for selecting at time intervals different portions of different recurrent signals, means for feeding said train of signals to said switching means and means for feeding said different portions from said switching means to said integrating device, so as to set up oscillations therein, which oscillations are integrated to develop oscillations having substantially the same envelope waveform as at least a part of the envelope waveform of said recurrent oscillatory signals, and means for continuously changing in the same sense the timing of the intervals during which said portions of said signalsare fed to said integrating device during a time period long compared with the time of recurrence of said recurrent signals, whereby the minimum ratio of the amplitude of said recurrent signals to the amplitude of any random interference superimposed thereon is increased in said integrating device.

6. Apparatus according to claim 5, comprising rectifying means, means for feeding said oscillations developed in said integrating device to said rectifying means, a further integrating device, and means for feeding the rectified signals in said rectifying means to said further integrating device whereby the minimum ratio of the amplitude of said rectified signals and any random interference is increased,

'7. Apparatus for detecting a reflecting object comprising transmitting means for radiating short bursts of oscillatory signals having substantially the same envelope Waveform, receiving means for receiving said signals after reection by said object, frequency changing means for deriving oscillatory signals of lower frequency from said signals received by said receiving means, means for feeding said received signals to said frequency changing means, an oscillatory circuit of low decrement tuned to said lower frequency, means for feeding said signals of lower frequency to said oscillatory circuit so as to set up oscillations therein, switching means for selecting during time intervals different portions of different signals, said switching means being interposed between said receiving means and said oscillatory circuit, and means for continuously changing in the same sense the timing of the intervals during which said portions of said signals are fed to said oscillatory circuit during a time period long compared with the time of recurrence of said bursts of signals.

8. Apparatus according to claim 7, in which said frequency changing means comprises means for rendering said lower frequency substantially independent of changes in the frequency of said received signals due to motion of said reiiecting object.

9. Apparatus for determining the position of a reflecting object, comprising transmitting means for radiating short bursts of oscillatory signals having substantially the same envelope waveform, receiving means for receiving said signals after reflection by said object, frequency changing means for deriving oscillatory signals,

of lower frequency from said signals received by said receiving means, means for feeding said received signals to said frequency changingv means, an oscillatory circuit of low decrement 13 tuned to said lower frequency, means for feeding said signals of lower frequency to said oscillatory circuit so as to set up oscillations therein, switching means for selecting during time intervals different portions of different signals, said switching means being interposed between said receiving means and said oscillatory circuit, means for continuously changing in the same sense the timing of the intervals during which said portions of said signals are fed to said oscillatory circuit during a time period long compared with the time of recurrence of said bursts of signals so as to develop in said oscillatory circuit an oscillatory signal having substantially the envelope waveform as at least a part of the envelope waveform of said received signals, a further receiving means spaced from said first-mentioned receiving means for receiving said signals after reflection .by said object, further frequency changing means for deriving oscillatory signals of lower frequency from said signals received by said further receiving means, means for feeding said received signals to said further frequency changing means, a further oscillatory circuit of low decrement tuned to said last-mentioned lower frequency, means for feeding said signals of said last mentioned lower frequency to said further oscillatory circuit so as to set up oscillations therein, further switching means for selecting during time intervals different portions of different signals, said further switching means being interposed between said further receiving means and said further oscillatory circuit, further means for continuously changing in the same sense the timing of the intervals during which said portions of said signals are fed to Isaid further oscillatory circuit during a time period long compared with the time of recurrence of said bursts of signals so as to develop in said further oscillatory circuit` an oscillatory signal having substantially the same envelope waveform as at lea-st a part of the envelope waveform of said signals received by said further receiving means, means for adjusting and measuring the difference in timing between said time intervals during which said different portions of said signals received by said receiving means and said further receiving means are fed to said oscillatory circuit and said further oscillatory circuit respectively whereby when the timings of said intervals are so adjusted that the oscillations developed in said oscillatory circuit and said further oscillatory circuit have substantially the same envelope waveform, the difference 14 in timing between said intervals is related to the bearing of said reflecting object from an imaginary line joining said receiving means and said further receiving means.

10. Apparatus for determining the distance of a reiiecting object comprising pulse generating means for generating a train of recurrent pulses having substantially the same waveform, transmitting means for radiating oscillatory signals, means for feeding said pulses to said transmitting means so as to cause the radiation of said oscillatory signals only when said pulses are fed to said transmitting means, receiving means for receiving said signals after reection by said object, frequency changing means for deriving oscillatory signals of lower frequency from said signals received by said receiving means, means for feeding said received signals to said frequency changing means, an oscillatory circuit of low decrement tuned to said lower frequency, means for feeding said signals of lower frequency to said oscillatory circuit so as to set up oscillations therein, switching means for selecting different portions of dierent signals, said switching means being interposed between said receiving means andv said oscillatory circuit, an adjustable time delay circuit, means for feeding pulses from said pulse generating means to said time delay circuit, means for changing'the timing of said pulses continuously in the same sense during a time period long compared with the time of recurrence l a part of the -envelope waveform of said radiated signals, the distance of said object is related to the time delay introduced by said time delay circuit.

ALAN DOWER BLUMLEIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,055,883 Terry Sept. 29, 1936 2,225,524 Percival Dec. 17, 1940 2,208,349 Ulbricht July 16, 1940 

